what are the most bombastic classical pieces?

Wagner's operas. All of them

Symphony No.1 in D minor ''Gothic'' by Havergal Brian

les preludes, it reaches for greatness and goes  a loud splat

beethoven 9 it gets to the mountain top

berlioz requiem.spiritual overload

le sacre du printemps,diesel train horns approacheth to runya down  firebird too

Rachmaninoff's Moment Musicaux no.4 is an explosion of anger and ultra fast note cascades for the left hand, powerful passages written as FFF (very very loud) and an aggressive theme.

Since "bombast" and "bombastic" refer to overblown speech or writing, no classical music piece can be bombastic. Perhaps you mean which pieces are the noisiest. There's plenty of candidates for that.

I don't think of classical music as being bombastic.

4'33. Absolutely nothing can come close to the ballsiness it took to write that.

Les Préludes by Liszt.

1812 Overture            

Rite of Spring is the one that springs to mind.  It's epic.